[#2750] Remove form_to_db_schema_options() from ExampleIDatasetForm

It was causing problems, is not supposed to be overridden.
[#2750] Remove db_to_form_schema_options() from ExampleIDatasetForm

Is not supposed to be overridden, not part of the IDatasetForm
interface.
[#2750] Fix example_idatasetform's check_data_dict

It had the wrong number of params, this was probably causing some crashes and test fails.
Merge remote-tracking branch 'okfn/2750-add-docs-and-examples-for-idatasetform-and-igroupform' into 2750-add-docs-and-examples-for-idatasetform-and-igroupform

Conflicts:
	ckanext/example_idatasetform/plugin.py